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Distributed Systems CS 3850 Soufiane Noureddine Lectures MWF 14:00 – 14:50 (PE207D) Office Hours MW 11:00 – 12:00 (C520) Introduction Chapter 1 Computer Networks
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Chapter 1
A collection of computers communicating through an underlying network is called a computer network (in contrast to a single-processor system)
Local Area Network: LAN
10 to 1000 Mb/sec
Wide Area Network: WAN
64 kbps to gigabits/sec
Definition of a Distributed System (2)
“You know you have one when the crash of a computer you never heard of stops you from getting any work done.”
L. Lamport
1.1
A distributed system organized as middleware.Note that the middleware layer extends over multiple machines.
Characteristics of a distributed System
Differences between computers are hidden
Communication between computers is hidden
Internal organization of the system is hidden
Single system image: interaction with the system is independent from location (and time)
Ease of extension
High availability
Examples of distributed Systems
Different forms of transparency in a distributed system.
Degree of transparency: More transparency means less performance
In reality: Systems are transparent only to certain degree
(e.g. Telecom)
Clients
Software Developers
e.g. Company
Applications
Operating System & Hardware
Network Provider
(e.g. Telecom)
Clients
Software Developers
e.g. Company
Applications
AEM: Availability Enhancing Middleware
Operating System & Hardware
Network Provider
A system is scalable when it is easy to extend without loss of performance.
Example: Move part of computation from server to client (e.g. Java applet)
1.4
Ad b) Use of distribution
1.5
An example of dividing the DNS name space into zones.
Clientserver Z1: nl.vu.cs.fluit
Server Z1 Client: address of Z2
Clientserver Z2: vu.cs.fluit
…
Uniprocessor Operating Systems
1.11
Separating applications from operating system code through
a microkernel.
Multiprocessor Operating Systems (1)
monitor Counter {
private:
int count = 0;
public:
int value() { return count;}
void incr () { count = count + 1;}
void decr() { count = count – 1;}
}
A monitor to protect an integer against concurrent access.
Multiprocessor Operating Systems (2)
monitor Counter {
private:
int count = 0;
int blocked_procs = 0;
condition unblocked;
public:
int value () { return count;}
void incr () {
if (blocked_procs == 0)
count = count + 1;
else
signal (unblocked);
}
void decr() {
if (count ==0) {
blocked_procs = blocked_procs + 1;
wait (unblocked);
blocked_procs = blocked_procs – 1;
}
else
count = count – 1;
}
}
A monitor to protect an integer against concurrent access, but
blocking a process.
Multicomputer Operating Systems (3)
Relation between blocking, buffering, and reliable communications.
Distributed Shared Memory Systems (1)
Distributed Shared Memory Systems (2)
Page size: small more communication overhead
large less communication, but false sharing may occur
1.18
False sharing of a page between two independent processes.
Problem with DSM: not as efficient as expected
Network Operating System - NOS (1)
1-19
General structure of a network operating system.
Main features: Independent operating systems
Services for accessing remote resources
Distributed operating system:
Fully transparent
For homogeneous systems computers are not independent More secure, but less scalable and less open
Network operating system:
Not transparent
For heterogeneous system computers are independent
Easier to extend (e.g. adding a new node in the Internet)
Both do not really qualify as a distributed system!!!
Solution: Middleware atop of a NOS hiding heterogeneity and improving transparency
1-22
General structure of a distributed system as middleware.
Middleware Services: rather a functionally complete set of services in order to hide heterogeneity, direct access to NOS is discouraged.
1.23
In an open middleware-based distributed system, the protocols used by each middleware layer should be the same, as well as the interfaces they offer to applications.
implementations of the middleware should not use the NOS-provided protocols
A comparison between multiprocessor operating systems, multicomputer operating systems, network operating systems, and middleware-based distributed systems.
An Example Client and Server (1)
The header.h file used by the client and server.
An Example Client and Server (2)
A sample server.
1-28
The general organization of an Internet search engine into three different layers
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